Production method of iron carbide

ABSTRACT

Provided is a method for efficiently producing iron carbide depending on a particle size of an iron-containing material or the progress of reaction. In a fluidized bed reactor  7 , a coarse iron ore is fluidized in blocks  8   a  to  8   e , a fine iron ore is fluidized in blocks  9   a  to  9   d , and a flow rate of a reaction gas supplied to the blocks for the fine iron ore is regulated by a flow regulating valve  11.

TECHNICAL FIELD

The present invention relates to a method for producing iron carbidesuitable for a raw material for iron making and steel making whichcomprises iron carbide (Fe₃C) as the main component, for example, a rawmaterial for steel making which is used in an electric furnace and thelike

BACKGROUND ART

The production of steel normally comprises the steps of converting ironore to pig iron using a blast furnace, and thereafter converting the pigiron into steel using an open hearth furnace or a converter. Such atraditional method requires large amounts of energy and large-scaleequipment, and has a high cost. Therefore, for a small-scalesteel-making, a method comprising the steps of directly converting ironore into raw materials used in the steel-making furnace, and convertingthe raw material into steel using an electric furnace and the like hasbeen used. With respect to direct steel making process, a directreduction process has been used to convert iron ore into reduced iron.However, the reduced iron produced by the direct reduction process ishighly reactive and reacts with oxygen in the air to generate heat.Therefore, it is necessary to seal the reduced iron with an inert gas,or by some other measures, during transportation and storage of thereduced iron. Accordingly, iron carbide (Fe₃C) containing acomparatively high iron (Fe) content, and which has a low reactionactivity and can be easily transported and stored, has recently beenused as the iron-containing material for steel making in an electricfurnace and the like.

Furthermore, an iron-making or steel-making material containing ironcarbide as the main component is not only easy to be transported andstored, but also has the advantage that the carbon combined with ironelement can be used as a source of energy in an iron-making orsteel-making furnace, and can be used as a source to generatemicrobubbles which reduce nitrogen in the steel-making bath. Therefore,raw materials for iron making or steel making containing iron carbide asthe main component recently have attracted special interest.

According to a conventional method for producing iron carbide, a fineiron ore is charged into a fluidized bed reactor or the like, and iscaused to react with a gas mixture comprising a reducing gas (e. g.,hydrogen gas) and a carburizing gas (e. g., methane gas and the like) ata predetermined temperature. Thus, iron oxides (e. g., hematite (Fe₂O₃),magnetite (Fe₃O₄), wustite (FeO)) in iron ore are reduced and carburizedin a single process (which means a process performed by simultaneouslyintroducing a reducing gas and a carburizing gas to a single reactor).This reaction is performed by the following overall reaction formula.

3Fe₂O₃+5H₂+2CH₄→2Fe₃C+9H₂O

The prior art in the field of the present invention has been described,for example, in the publication of the Japanese translation ofInternational Patent Application No. 6-501983, for example.

In order to easily understand the present invention, an example of anapparatus for producing iron carbide according to the prior art will bedescribed below. For example, an apparatus shown in FIG. 1 has beenknown. With reference to FIG. 1, the reference number 1 denotes afluidized bed reactor. Fluidized bed reactor 1 has a bottom part towhich a line 2 for supplying reaction gases (a reducing gas and acarburizing gas) is connected, and a top part to which a line 3 fordischarging the gas after reaction is connected. The reference number 4denotes a preheating furnace. A fine iron ore fed to preheating furnace4 is subjected to a preheating treatment for a predetermined time inpreheating furnace 4. Then, the preheated iron ore is fed into fluidizedbed reactor 1 through a line 5, and is subjected to a reducing andcarburizing reaction for a predetermined time at a predeterminedreaction temperature and reaction pressure in fluidized bed reactor 1.Thus, iron carbide product is discharged from a line 6.

In the case where a particle size distribution of the iron ore is wide,it is difficult to proceed the reaction efficiently. The reason is asfollows. In order to proceed the reaction efficiently, it is preferablethat a velocity of a fluidized gas should be comparatively increased influidized bed reactor 1 if major particle size of the iron ore is large(coarse) but fine ores should be blown off, and that the velocity of thefluidized gas should be comparatively decreased in fluidized bed reactor1 if major particle size of the iron ore is small (fine) but coarse oresshould not be fluidized. There are preferable process conditionsdepending on respective particle sizes. Furthermore, a moving bedreactor is preferable for the iron ore having a large particle size. Agas for proceeding the reaction can easily pass through a gap in thelarge uniform particle size. An increase in the flow rate of thefluidized gas for fluidization causes generation of fine-sized iron oreby a further friction of particles and is disadvantageous to a yield ofiron ores.

As indicated in the overall reaction formula, a particle of Fe₂O₃ isconverted into a particle of Fe₃C having about ¾ of an original weight.Furthermore, the fine iron ores rub against each other duringfluidization so that their particle sizes are gradually reduced. Takingit into consideration that the weight of the fluidized material (fineiron ore) is gradually reduced as the reaction proceeds, it ispreferable that the velocity of the reaction gas to be supplied to thefluidized bed reactor should be comparatively increased in the formerhalf of the reaction and be comparatively decreased in the latter halfof the reaction in order to proceed the reaction efficiently. Sincethere are proper process conditions according to the progress of thereaction, it is not preferable that the reducing reaction and thecarburizing reaction should be performed under the same processconditions in the fluidized bed reactor.

In consideration of the above-mentioned problems of the prior art, it isan object of the present invention to provide a method for efficientlyproducing iron carbide depending on a particle size of aniron-containing material or the progress of reaction.

DISCLOSURE OF INVENTION

In order to attain the above-mentioned object, the present inventionprovides a method for efficiently producing iron carbide by classifyinga fine iron-containing material for iron making into several gradesaccording to a particle size, and by reducing and carburizing theiron-containing material corresponding to the respective particle sizes.

A first aspect of the present invention is directed to a method forproducing iron carbide comprising the steps of classifying a fineiron-containing material for iron making into several grades accordingto a particle size, and reducing and carburizing each iron-containingmaterial belonging to each grade. According to the first aspect of thepresent invention, it is possible to treat a fine iron- containingmaterial for iron making which has a wide particle size distribution. Byselecting the process conditions depending on a particle size, ironcarbide can be produced efficiently.

A second aspect of the present invention is directed to the method forproducing iron carbide according to the first aspect of the presentinvention, wherein the fine iron-containing material for iron making isclassified into several grades according to a particle size afterpreheating. According to the second aspect of the present invention, thefollowing effects can be obtained in addition to the above-mentionedeffects. More specifically, if it is difficult to classify a wet iron-containing material, a classifying operation itself can easily beperformed because the classification is performed in a dry state afterpreheating or a first stage reaction process. Furthermore, the presentinvention is suitable for treating such iron-containing material as thefine iron ore generated from a raw material which is easily broken byheat does not become a by-product but a product.

A third aspect of the present invention is directed to the method forproducing iron carbide according to the second aspect of the presentinvention, wherein reducing and carburizing reaction process comprise afirst stage reaction process for performing a part of reducing reactionand then a second stage reaction process for performing further reducingand carburizing reaction. According to the third aspect of the presentinvention, the following effects can be obtained in addition to theabove-mentioned effects. Various countermeasures can be taken for eachprocess which cannot be performed by the method for producing ironcarbide in a single reactor process according to the prior art. Thus, aflexible process can be attained. Consequently, a conversion rate and areaction speed can easily be controlled. Furthermore, the energyconsumed in the generation of the by-product can be recoveredeffectively.

A fourth aspect of the present invention is directed to a method forproducing iron carbide, comprising the steps of classifying a fineiron-containing material for iron making into several grades accordingto a particle size after a first stage reaction process for reducing apart of the iron-containing material, and carrying out a second stagereaction process for performing further reducing and carburizingreaction for each iron-containing material belonging to each grade.According to the fourth aspect of the present invention, the method ofthe present invention is suitable for treating the iron-containingmaterial which become easily finer by a reducing reaction.

A fifth aspect of the present invention is directed to a method forconverting a fine iron-containing material for iron making into ironcarbide by a fluidized bed reactor having a fluidized bed part dividedinto several compartments by partition walls, comprising the steps ofdividing the compartments into two portions for coarse and fineiron-containing materials respectively, and reducing and carburizing thecoarse iron-containing materials in the one portion and the fineiron-containing materials in the other portion. According to the fifthaspect of the present invention, the supply of the raw material can betreated in one reactor in which fine and coarse ores are reactedseparately under proper conditions for each ores. Thus, utilization ofreaction gas is optimized and efficient process is achieved.

In the present invention, the iron-containing material for iron makingis an iron ore or a dust or the like which is generated from an ironmaking process comprising at least one of iron oxides such as hematite,magnetite and wustite and iron hydroxides such as ferrous hydroxide andferric hydroxide or the mixtures thereof of more than two as the maincomponent.

According to the present invention having the above-mentionedconstitution, the fine iron-containing material for iron making whichhas a wide particle size distribution can be classified into severalgrades according to the particle sizes, and process conditions (reactiontemperature, reaction time, a gas flow rate and the like) correspondingto the respective particle sizes can be selected for an iron-containingmaterial belonging to each grade. Consequently, iron carbide can beproduced efficiently.

Some iron-containing materials are easily broken by heat. Such materialsare classified after preheating, and process conditions depending oneach particle size are selected after the classification. Consequently,iron carbide can be produced efficiently.

Furthermore, by applying a two-stages process for carrying out a firststage reaction process performing a part of reducing reaction and thena. second stage reaction process performing further reducing andcarburizing reaction, a gas used in the first stage reaction process canbe an optimum composition for only the reducing reaction, and a gas usedin the second stage reaction process can be an optimum composition forthe further reducing and carburizing reaction. By applying thetwo-stages process, in the reduction and carburization (conversion intoiron carbide) of the iron-containing material, a reaction speed can beincreased and a reaction time (a time required for converting theiron-containing material into iron carbide) can be shortened as comparedwith a process for producing iron carbide in a single process.

Some iron-containing materials become easily finer by a reducingreaction. Such materials are classified after partial reduction, and thereducing and carburizing reaction conditions corresponding to eachparticle size are selected after the classification. Consequently, thereaction can be performed efficiently.

In the fluidized bed reactor having a fluidized bed part divided intoseveral compartments partition walls, the compartments are classifiedinto two portions for iron-containing materials comprising coarse andfine particles, and reducing and carburizing the iron-containingmaterials comprising coarse and fine particles are separately carriedout. Consequently, the supply of the raw material and the discharge ofthe products can be continuously performed, and the fluidization canuniformly be carried out for both coarse and fine particles, and acontact area between the reaction gas and the raw material can bedesigned properly for fine particles so that the reaction time can beshortened.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a schematic flow according to a prior art of anapparatus for producing iron carbide;

FIGS. 2 (a) and 2 (b) are diagrams showing examples of the processperformed when producing the iron carbide according to the presentinvention, respectively;

FIG. 3 is a diagram showing another example of the process performedwhen producing the iron carbide according to the present invention;

FIG. 4 is a chart showing a range of fluidization based on a particlesize and a gas flow rate; and

FIG. 5 (a) is a side sectional view showing a fluidized bed reactorsuitable for carrying out a method according to the present invention,and FIG. 5 (b) is a sectional view taken along the line V—V in FIG. 5(a).

BEST MODE FOR CARRYING OUT THE INVENTION

The case where a method according to the present invention is applied toa fluidized bed reactor will be described below.

(1) Timing of Classification

As described above, some iron-containing materials are easily broken byheat or become easily finer by a reducing reaction. For example, therate of generation of a fine iron ore having a size of 70 μm or less isgenerally varied according to the kind of the raw material iron ore(from 0.1 mm to 1.0 mm) depending on the circumstances of the treatmentto which the iron ore is subjected as shown in Table 1.

TABLE 1 Kind of Iron Ore A B C D After preheating  1% 8% 0% 12% Afterreduction 12% 2% 5%  4% After carburization  5% 1% 2%  1%

As will be described below, the optimum process conditions can be takenby applying a timing of classification suitable for the kind of the ironore. (a) In the case where the iron ore (ore “B” and “D”) which iseasily broken by heat is used as a raw material, it is preferable thatthe process shown in FIGS. 2 (a) and 2 (b) should be applied. (b) In thecase where the iron ore (ore “A”) which become easily finer by areducing reaction is used as the raw material, it is preferable that theprocess shown in FIG. 3 should be applied.

In FIGS. 2 (a) and 2 (b) and FIG. 3, while coarse and fine iron ores canbe separately treated in respective reactor, they can simultaneously betreated in one reactor if the reactor has the configuration described in(3). Furthermore, the classification can be set to two or more dependingon the particle size distribution of the iron ore.

(2) Selection of Classification Size

A classification size for division into coarse and fine iron ores basedon the particle size distribution of the iron ore can be obtained asshown in FIG. 4, for example. FIG. 4 shows a range of fluidization,wherein a horizontal axis indicates a particle size (dp: logarithmicrepresentation) and a vertical axis indicates a superficial speed (u:logarithmic representation). The line A indicates a lower limit of thefluidization. Below the line A, the speed is not enough to fluidize theiron ore in the reactor. The line B indicates a limit of a blowing speed(terminal speed). Above the line B, the speed is too high so that theiron ore is blown off, and similarly, the iron ore can be neitherfloated nor fluidized.

As shown in FIG. 4, if the particle size distribution of the iron oreranges from 0.1 to 1.0 mm, the iron ore can be floated and fluidizedunder the same process conditions. Because, gas speed at A1 is lowerthan that at B1. However, if the particle size distribution of the ironore is wide, for example, the iron ore comprises a fine iron ore havinga particle size of 0.05 to 0.5 mm and a coarse iron ore having aparticle size of 0.5 to 5 mm, it is preferable that the reaction shouldbe performed for the fine iron ore and the coarse iron ore separately inorder to cause the reaction to proceed efficiently and that a boundarysize for division into “fine” and “coarse” should range from 0.2 to 0.8mm. If the coarse iron ores having particle sizes of 5 to 7 mm or moreis major, it is preferable that a moving bed reactor should be used.

(3) Equipment for Classification

In the case where the iron ore is classified on the basis of theparticle size distribution, the coarse iron ore and the fine iron orecan be treated in separate reactors to take operating conditionsdepending on the particle size. However, the coarse iron ore and thefine iron ore can be simultaneously treated in one reactor if thereactor has a configuration shown in FIG. 5. More specifically,fluidized bed reactor 7 is divided into compartments 8 a to 8 e for thecoarse iron ore and compartments 9 a to 9 d for the fine iron ore, aflow regulating valve 11 is provided on a line 10 for a reaction gas tobe supplied to the compartments of the fine iron ore, and the fine ironore in a gas discharged from the reactor is caught by a cyclone 12. Thefine iron ore is returned to the compartments 9 a to 9 d.

According to the reactor having the above-mentioned configuration, it isalso possible to simultaneously treat the fine iron ore and the coarseiron ore by using a single reactor having the same inlet gas compositionaccording to the following method. By adjusting flow regulating valve11, it is possible to control the flow rate of the reaction gas to havebest fluidization of the fine and coarse iron ore. In this case, it ispreferable that a flow rate (=superficial velocity) of the reaction gasfor the fine iron ore should be decreased to avoid blowing off fine ironore and a bed height (Hff) of the fine iron ore should be set smallerthan a bed height (Hfc) of the coarse iron ore to have the same contact(gas and ores) time. The fine iron ore tends to have a reaction speedwhich is a little higher than the coarse iron ore as shown in Table 2.Table 2 indicates a reduction rate, wherein a Gas vs. Solid ratio is setto 0.059 kg/SLM, a reaction temperature is set to 63° C. a reactionpressure is set to 4 to 5 atm, and a hydrogen concentration in thereaction gas is set to 65 to 80%. Fine iron ore (Z) has a reduction ratewhich is a little higher than that of coarse iron ore (X, Y).

SLM designates Standard Liter per Minute (1 liter in a normalcondition/minute).

Therefore, in case that fine and coarse ores are reacted in one reactor,fine ores is too much acted to react coarse ores properly.

TABLE 2 Reaction Time Kind of Iron Ore Particle Size (mm) 30 mins. 60mins. X  0.1 to 0.5 26% 38% Y ″ — 45% Z 0.01 to 0.1 31% 48%

According to the present invention, the fine iron ore and the coarseiron ore are reacted separately in the same fluidized bed reactor.Consequently, an amount of a gas to be used per unit weight of iron orecan be reduced and a residence time in the reactor can be shortened.Thus, it is possible to produce iron carbide economically andefficiently.

The classified iron ore of finer particles can be not only used forproducing iron carbide but also utilized as an auxiliary material forproducing cement or feed material after granulation.

INDUSTRIAL APPLICABILITY

Since the present invention has the above-mentioned constitution, theapparatus according to the present invention is suitable for theapparatus to efficiently produce iron carbide depending on a particlesize of an iron-containing material or the progress of reaction.

What is claimed is:
 1. A method for producing iron carbide comprisingthe steps of: classifying a fine-grained iron-containing material forproducing iron carbide into several grades according to a particle size;and separately reducing and carburizing each iron-containing materialbelonging to each grade, wherein the fine-grained iron-containingmaterial for producing iron carbide is classified into several gradesaccording to a particle size after preheating.
 2. The method forproducing iron carbide according to claim 1, wherein reducing andcarburizing reaction processes are performed in two stages comprisingthe steps of: a first stage reaction process for performing a part ofreducing reaction and then a second stage reaction process forperforming further reducing and carburizing reaction.
 3. A method forproducing iron carbide comprising the steps of: classifying afine-grained iron-containing material for producing iron carbide intoseveral grades according to a particle size after a first stage reactionprocess for partial reducing of the iron-containing material; andcarrying out a second stage reaction process for performing furtherreducing and carburizing reaction separately for each iron-containingmaterial belonging to each grade.
 4. A method for converting afine-grained iron-containing material into iron carbide by a fluidizedbed reactor having a fluidized bed part divided into severalcompartments by partition walls, characterized in that the compartmentsare divided into two portions for coarse iron-containing materials andfine-grained iron-containing materials respectively, and thecoarse-grained iron-containing materials have a particle size of 0.5 to5 mm and the fine-grained iron-containing materials have a particle sizeof 0.05 to 0.5 mm, and the method comprising the steps of reducing andcarburizing the coarse-grained iron-containing materials in the oneportion of the reactor and the fine-grained iron-containing materials inthe other portion of the reactor.